Make a multifeed LNB bracket for your satellite dish

 Author:   Posted on:   Updated on:  2019-08-14T20:07:55Z

A satellite dish can receive multiple satellites if you add more than one LNB to it. This requires a LNB bracket. Build it yourself and learn to place LNBs in a multifeed system depending on satellites you want to receive.

Although a satellite dish is designed to receive signal coming from a single direction, by placing additional LNBs near the main one you will be able to receive more than one orbital position. Thus, each LNB will receive signal from a different satellite. You can receive in this way signal from satellites placed at near orbital positions. Having multiple LNBs fitted to a dish is called multifeed and it comes with advantages and disadvantages. The main advantage over a motorized dish is the fact that it is a fixed installation and you can change channels instantly without waiting for the antenna to move. However, the additional LNBs that are not placed in the middle focal point will receive lower quality signal than the main one.

Brackets for multiple LNBs are commercially available, but I found them rather... rigid. Most of them are for 2 or 4 LNBs and don't allow too much fiddling to get the best signal quality. Let's try to make that bracket. It must keep a constant focal distance and should follow the inverted reflection of Clarke orbit. The focal distance depends on antenna and orbit reflection depends on dish size, receiving location and satellite positions. The distance between LNBs is proportional with dish diameter. You will learn how these parameters affect the LNB bracket shape and size and how you can calculate distance between LNBs.

Multifeed dish with 8 LNB

Photo courtesy of Paul Lucas on Flickr

There are two types of bracket configuration you may attempt to build. LNBs must attach to it in a flexible way to find the best signal quality so you must figure a way to do this too. I'll give you some examples throughout this post.

Multifeed LNB holder

The two types of holders you can build

The first type (top) uses a metal sheet bent as an arc shape and it sits vertically with the LNBs attached as shown. The second type (bottom) is an arc shape trimmed sheet that sits horizontally. I would recommend this second type because it is easier to adjust. You can make the bracket from 1 - 2 mm thick aluminum sheet or 0.5 - 1 mm stainless steel. Or any material that you can cut and bend and does not get deformed by LNBs weight or wind, snow etc. If you choose the horizontal type, the parameter you take into account for cutting the metal sheet is the focal distance (it's dish specific). For the second type, you must follow the orbit projection.

Focal distance

The holder must maintain a constant focal distance for the side LNBs. The focal distance is a dish parameter that you should know from its specifications. If you don't, you can measure it. It is the distance from the point on the dish where the LNB looks at and the original LNB holder. The best way to measure it is to insert a 4 cm diameter PVC pipe through the LNB holder and see where it points to. Then measure its length. No matter what kind of dish you have, the pipe should point somewhere on the vertical middle axis.

How to measure the focal distance of an offset dish

How to measure the focal distance of an offset dish

Now, that you know the focal distance you can make a template for your bracket. You must draw an arc with a radius equal to the focal distance. You can do this with an inextensible wire and a pencil by holding one end of the wire still on the table and drawing with the pen while tied by the wire. Or you can use a drawing software and print the result.

Holder drawing in Inkscape - screenshot

Inkscape example

Inkscape can be used. Draw three circles: one with the focal distance radius and the other two with a radius smaller by 1 cm and greater by 1 cm. Align the circles in order to "catch" an arc on the page then print it. This is the template for a horizontal type bracket, but can be used for vertical type too, if you bend your piece following the circle (arc) of interest.

Up-view of the dish and holder

The holder should be equally spaced with regard to the center of the dish.

The bracket should have a width of 2 cm and a length of 40 cm to have enough to play with. It's up to you how you attach it to the dish arm, but keep in mind that you must take into account each LNB holder (I will detail later how you can build those).

Orbit

In the vertical plane, the bracket should be a small-scale representation of the Geostationary orbit. This differs depending on what satellite positions you want to receive and what is your location longitude. If you plan to receive satellites that are situated both western and eastern with regard to your longitude, the bracket must be bent in "U" shape. For the other cases it can be straight and placed oblique as it will be shown below. Imagine you sit behind the dish and look at the following drawing.

Multifeed configurations as seen from behind the dish

Multifeed configurations as seen from behind the dish

Let's take some practical examples:

  • You live at 15° E and you want to receive 13° E, 5° E and 1° W. All these are Western satellites with respect to chosen location. The highest satellite (13° E and 16° E can be considered as such here) is at the edge of the holder. No matter which will be the center LNB (although it is recommended to be on 5° E), you're in situation B.
  • You live at 15° E and want to receive 19° E, 13° E and 5° E. If the center LNB is on 13° E, you're in situation A. If center is on 19° E, you're in situation E. Otherwise, if center is on 5° E, you're in situation D.
  • You live at 35° E and want to receive 45° E and 52° E. Both are Eastern satellites. You're in situation C.

Situations A, D and E require a bent bracket, in two planes. If you built the vertical one, now you can't do anything. This is why I recommended the horizontal piece bracket. The inclination angle of the bracket increases (goes towards vertical) as you approach equator. If you live at equator, the LNB holder is vertical, meaning you can only add LNBs over the top of the central one (with an offset dish).

An useful tool for this is the calculator software SatHunter. Note that this is abandoned shareware and you only have 10 tries to calculate your multifeed. Don't forget to set your location correctly.

SatHunter location settings

SatHunter location settings

Then go to Multifeed tab and set your antenna focal length and choose the satellites of interest. Note that as opposed to what I have described, this software shows you the front of the dish! The screenshot shows the first example from above.

Multifeed calculations in SatHunter

Multifeed calculations in SatHunter

You can download the last released version of SatHunter from here. Some of you may want to make the calculations without relying on SatHunter. If so, pick two satellites you want to receive and write down their azimuth and elevation (use DishPointer to find the exact parameters for your location). The formula is this:

Multifeed formula to calculated distance between LNBs

LNB attaching

You must attach LNBs to your homemade bracket. Here is what I suggest. You can make these holders from metal sheets with a thickness of 0.5-1.5 mm.

How to attach LNB to the holder

How to attach LNB to the holder

But where on the bracket do you attach them? The best way is trial and error. Connect the LNB to the receiver (via a DiSEqC switch). Note that the antenna should be pointed to a satellite (as good as possible) with the central LNB and the bracket mounted! Now take the second LNB in your hand and move it across the bracket until you get the best signal. If you built the horizontal type bracket, slightly bend it up and down to improve signal quality. When you're satisfied, mark the bracket and drill the required holes to fix the LNB. And one last thing, the holder can stand below the LNB as shown, or on top of them, which is actually better if your holder is "U" shaped (situation A).

3 comments :

  1. Many thanks for you time to explaining it very clear and easy to understand.

    ReplyDelete
  2. Awesome, I'm just knocking up a bracket with LNB holders in Fusion360 based on your explanation. :D
    I'll be 3d printing the parts.

    ReplyDelete

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